<s>
IPv4	O
Residual	O
Deployment	O
(	O
4rd	B-Protocol
)	O
is	O
an	O
IPv6	B-Protocol
transition	I-Protocol
mechanism	I-Protocol
for	O
Internet	O
service	O
providers	O
for	O
deployment	O
of	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
6	I-Protocol
(	O
IPv6	B-Protocol
)	O
,	O
while	O
maintaining	O
IPv4	O
service	O
to	O
customers	O
.	O
</s>
<s>
IPv4	B-Protocol
Residual	I-Protocol
Deployment	I-Protocol
has	O
three	O
main	O
features	O
:	O
</s>
<s>
Mesh	O
topology	O
:	O
between	O
two	O
endpoints	O
,	O
IPv4	O
packets	O
take	O
the	O
same	O
direct	O
routes	O
as	O
IPv6	B-Protocol
packets	O
.	O
</s>
<s>
Shared	O
IPv4	O
addresses	O
:	O
to	O
deal	O
with	O
the	O
unavoidable	O
IPv4-address	O
shortage	O
,	O
several	O
customers	O
can	O
be	O
assigned	O
a	O
common	O
IPv4	O
address	O
,	O
with	O
disjoint	O
TCP/UDP	O
port	O
sets	O
assigned	O
to	O
each	O
(	O
an	O
application	O
of	O
the	O
general	O
A+P	B-Protocol
model	O
of	O
RFC	O
6346	O
)	O
.	O
</s>
<s>
Stateless	O
operation	O
:	O
conversions	O
of	O
IPv4	O
packets	O
into	O
IPv6	B-Protocol
packets	O
at	O
domain	O
entry	O
,	O
and	O
the	O
reverse	O
at	O
domain	O
exit	O
,	O
are	O
stateless	O
(	O
i.e.	O
,	O
one	O
where	O
no	O
per-customer	O
state	O
is	O
needed	O
in	O
domain	O
edge	O
nodes	O
)	O
.	O
</s>
<s>
Full	O
IPv4-fragmentation	O
transparency	O
:	O
with	O
this	O
feature	O
,	O
support	O
of	O
the	O
path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
of	O
RFC	O
4821	O
,	O
recommended	O
in	O
RFC	O
6349	O
,	O
is	O
preserved	O
.	O
</s>
<s>
Applicability	O
of	O
IPv6	B-Protocol
packet	O
inspections	O
to	O
IPv4	O
:	O
when	O
traversing	O
IPv6-only	O
domains	O
,	O
converted	O
IPv4	O
packets	O
are	O
ordinary	O
IPv6	B-Protocol
packets	O
,	O
with	O
their	O
contents	O
unchanged	O
and	O
valid	O
in	O
IPv6	B-Protocol
.	O
</s>
<s>
Thus	O
,	O
IPv6	B-Protocol
filters	O
that	O
are	O
performed	O
within	O
the	O
IPv6-only	O
domain	O
,	O
e.g.	O
</s>
<s>
for	O
Access	O
Control	O
Lists	O
,	O
Web	B-Protocol
caches	I-Protocol
,	O
Deep	B-Application
packet	I-Application
inspections	I-Application
,	O
are	O
,	O
implicitly	O
and	O
automatically	O
,	O
effective	O
on	O
domain-traversing	O
IPv4	O
packets	O
.	O
</s>
<s>
If	O
an	O
ISP	O
wants	O
to	O
offer	O
residual	O
IPv4	O
service	O
across	O
an	O
IPv6-only	O
domain	O
,	O
and	O
provides	O
customer-premises	O
equipment	O
to	O
all	O
its	O
customers	O
of	O
this	O
domain	O
,	O
it	O
can	O
choose	O
any	O
of	O
MAP-E	O
,	O
MAP-T	O
,	O
or	O
4rd	B-Protocol
,	O
with	O
due	O
awareness	O
that	O
MAP-E	O
and	O
MAP-T	O
are	O
specified	O
in	O
standards-track	O
RFCs	O
,	O
while	O
4rd	B-Protocol
is	O
,	O
at	O
least	O
so	O
far	O
,	O
specified	O
in	O
an	O
experimental-track	O
RFC	O
(	O
see	O
the	O
History	O
section	O
below	O
)	O
:	O
the	O
chosen	O
mechanism	O
remains	O
purely	O
internal	O
to	O
each	O
domain	O
.	O
</s>
<s>
The	O
key	O
that	O
permits	O
to	O
combine	O
IPv4-fragmentation	O
transparency	O
with	O
IPv6	B-Protocol
deep	B-Application
packet	I-Application
Inspection	I-Application
in	O
a	O
single	O
design	O
is	O
the	O
use	O
of	O
a	O
reversible	O
packet	O
translation	O
at	O
Domain	O
Entries	O
and	O
Exits	O
.	O
</s>
<s>
This	O
is	O
possible	O
because	O
IPv6	B-Protocol
packet	O
headers	O
,	O
duly	O
complemented	O
with	O
their	O
Fragment	O
headers	O
whenever	O
needed	O
,	O
are	O
large	O
enough	O
to	O
encode	O
in	O
them	O
,	O
in	O
an	O
ad	O
hoc	O
way	O
detailed	O
in	O
RFC	O
7600	O
,	O
all	O
useful	O
IPv4-header	O
information	O
.	O
</s>
<s>
(	O
This	O
was	O
not	O
possible	O
in	O
6rd	B-Protocol
,	O
the	O
tunneling	O
mechanism	O
for	O
IPv6	B-Protocol
across	O
IPv4-only	O
domains	O
,	O
because	O
IPv4	O
headers	O
are	O
too	O
small	O
to	O
contain	O
all	O
IPv6-header	O
information	O
)	O
.	O
</s>
<s>
IP-layer	O
options	O
of	O
IPv4	O
are	O
not	O
supported	O
in	O
4rd	B-Protocol
,	O
but	O
without	O
practical	O
consequence	O
because	O
end	O
systems	O
are	O
already	O
adapted	O
to	O
the	O
fact	O
that	O
,	O
for	O
security	O
reasons	O
,	O
IPv4	O
IP-layer	O
options	O
are	O
filtered	O
by	O
many	O
routers	O
.	O
</s>
<s>
Another	O
issue	O
about	O
which	O
the	O
4rd	B-Protocol
specification	O
goes	O
beyond	O
those	O
of	O
MAP-E	O
and	O
MAP-T	O
concerns	O
fragmented	O
IPv4	O
datagrams	O
.	O
</s>
<s>
In	O
order	O
to	O
improve	O
user-perceived	O
performance	O
,	O
to	O
reduce	O
domain-entry	O
processing	O
,	O
and	O
to	O
reduce	O
attack	O
opportunities	O
,	O
the	O
4rd	B-Protocol
specification	O
includes	O
an	O
algorithm	O
whereby	O
received	O
fragments	O
of	O
large	O
datagrams	O
are	O
forwarded	O
one	O
by	O
one	O
on	O
the	O
fly	O
.	O
</s>
<s>
The	O
first	O
"	O
4rd	B-Protocol
"	O
specification	O
,	O
unlike	O
the	O
current	O
one	O
of	O
RFC	O
7600	O
,	O
used	O
IPv4	O
encapsulation	O
in	O
IPv6	B-Protocol
packets	O
,	O
the	O
only	O
known	O
tunneling	O
approach	O
at	O
that	O
time	O
to	O
ensure	O
complete	O
IPv4	O
preservation	O
across	O
IPv6-only	O
domains	O
.	O
</s>
<s>
It	O
was	O
the	O
first	O
proposal	O
that	O
combined	O
stateless	O
address	O
mapping	O
,	O
mesh	O
topology	O
,	O
and	O
A+P	B-Protocol
.	O
</s>
<s>
Another	O
stateless-mesh-A	O
+P	O
approach	O
was	O
next	O
proposed	O
,	O
called	O
dIVI	O
.	O
</s>
<s>
Instead	O
of	O
encapsulation	O
,	O
it	O
used	O
two	O
successive	O
translations	O
(	O
from	O
IPv4	O
to	O
IPv6	B-Protocol
and	O
then	O
conversely	O
)	O
,	O
based	O
on	O
the	O
existing	O
SIIT	O
one-way	O
translations	O
of	O
RFC	O
2765	O
.	O
</s>
<s>
Compared	O
to	O
encapsulation	O
,	O
it	O
had	O
the	O
advantage	O
of	O
making	O
IPv6	B-Protocol
packet	O
inspections	O
applicable	O
to	O
translated	O
UDP	O
and	O
TCP	O
IPv4	O
packets	O
,	O
but	O
,	O
due	O
to	O
limitations	O
of	O
SIIT	O
,	O
lacked	O
full	O
compatibility	O
with	O
IPv4	O
fragmentation	O
(	O
and	O
consequently	O
,	O
as	O
mentioned	O
above	O
,	O
compatibility	O
with	O
path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
recommended	O
in	O
RFC	O
6349	O
)	O
.	O
</s>
<s>
One	O
proposed	O
to	O
rename	O
the	O
4rd	B-Protocol
encapsulation	O
solution	O
as	O
MAP-E	O
,	O
to	O
rename	O
the	O
double	O
SIIT	O
translation	O
as	O
MAP-T	O
,	O
and	O
to	O
associate	O
them	O
into	O
a	O
combined	O
specification	O
named	O
MAP	O
.	O
</s>
<s>
The	O
idea	O
was	O
that	O
,	O
to	O
satisfy	O
the	O
standard	O
unicity	O
objective	O
,	O
a	O
specification	O
having	O
two	O
variants	O
(	O
among	O
which	O
a	O
choice	O
remains	O
needed	O
for	O
each	O
IPv6-only	O
domain	O
)	O
,	O
might	O
be	O
considered	O
as	O
equivalent	O
to	O
a	O
single	O
standard	O
.	O
</s>
<s>
The	O
other	O
was	O
based	O
on	O
discovery	O
that	O
,	O
as	O
mentioned	O
above	O
,	O
an	O
upgraded	O
IPv4-IPv6	O
double	O
translation	O
algorithm	O
was	O
possible	O
that	O
combined	O
applicability	O
of	O
IPv6	B-Protocol
packet	O
inspections	O
to	O
IPv4	O
,	O
like	O
MAP-T	O
,	O
and	O
full	O
compatibility	O
with	O
IPv4	O
fragmentation	O
like	O
MAP-E	O
.	O
As	O
the	O
"	O
4rd	B-Protocol
"	O
acronym	O
was	O
no	O
longer	O
used	O
for	O
the	O
encapsulation	O
solution	O
,	O
this	O
solution	O
was	O
named	O
4rd	B-Protocol
.	O
</s>
<s>
After	O
a	O
long	O
debate	O
,	O
the	O
Softwire	O
working	O
group	O
decided	O
,	O
in	O
August	O
2012	O
,	O
that	O
MAP-E	O
alone	O
would	O
be	O
standardized	O
,	O
and	O
that	O
work	O
could	O
continue	O
on	O
both	O
4rd	B-Protocol
and	O
MAP-T	O
,	O
but	O
only	O
as	O
experimental	O
.	O
</s>
<s>
Finally	O
,	O
in	O
December	O
2014	O
the	O
Softwire	O
working	O
group	O
changed	O
its	O
previous	O
decision	O
,	O
and	O
decided	O
to	O
put	O
MAP-T	O
on	O
Standards	O
Track	O
in	O
parallel	O
with	O
MAP-E	O
,	O
provided	O
a	O
note	O
in	O
the	O
MAP-T	O
RFC	O
would	O
signal	O
its	O
incompatibility	O
with	O
the	O
path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
of	O
RFC	O
4821	O
.	O
</s>
<s>
This	O
left	O
4rd	B-Protocol
alone	O
in	O
the	O
Experimental	O
category	O
(	O
yet	O
with	O
ISPs	O
 '	O
possibility	O
to	O
deploy	O
it	O
,	O
for	O
its	O
functional	O
advantages	O
,	O
in	O
domains	O
where	O
they	O
provide	O
customer	O
premises	O
equipment	O
to	O
all	O
their	O
customers	O
)	O
.	O
</s>
<s>
The	O
French	O
ISP	O
Free	O
is	O
deemed	O
to	O
have	O
deployed	O
4rd	B-Protocol
for	O
its	O
experiment	O
of	O
FTTH	B-Architecture
in	O
"	O
lesser-dense	O
areas	O
"	O
,	O
starting	O
from	O
December	O
2015	O
.	O
</s>
<s>
The	O
implementation	O
of	O
the	O
A+P	B-Protocol
model	O
implies	O
the	O
attribution	O
of	O
four	O
contiguous	O
port	O
ranges	O
to	O
different	O
customers	O
for	O
each	O
IPv4	O
address	O
.	O
</s>
<s>
Free	O
was	O
also	O
known	O
to	O
be	O
the	O
first	O
implementer	O
of	O
6rd	B-Protocol
.	O
</s>
